Mechanical couplings often require precision for applications in which the position of a machine is critical. For instance, mechanical precision in robotics is important for various purposes, for example, efficiency of mechanical components and accuracy of end-effector position. A common mechanical coupling in robotics utilizes gear couplings or gearboxes. Gearboxes can be particularly useful because electromagnetic transducers can deliver very low torques at very high speeds. In applications where bidirectional actuation with a gear reduction is necessary, however, backlash is a relevant concern.
Backlash is the amount of space between mating components of gears. By design, this space is built in to account for, inter alia, errors in gear manufacture and thermal expansion. In applications in which the gear couplings can be reversed, such as robotics, backlash of the mating components of the gears negatively affects the precision of the gear couplings. Moreover, robotic applications typically utilize gearboxes with multiple stages of gears. In such arrangements, backlash at each stage has a cascading effect, in that the gear ratio of each stage magnifies the backlash of the preceding stage. Therefore, in multi-stage gearboxes, the final stage of the gearbox is the most important for removing or reducing backlash, especially in applications in which the gear ratio of the final stage is large.
In standard gear trains, backlash is inevitable due to the over-constrained nature of gears. Care can be taken to significantly reduce backlash by tight tolerancing, but at significant cost. Other solutions to backlash have been developed. In one approach, harmonic drive gearboxes have no backlash, but are not resistant to shock loading, and are typically proprietary and expensive. In another approach, cycloidal gearboxes are another style of gearbox which remove backlash, and while they do not share the same durability issues as harmonic drives, they still tend to be proprietary and expensive.
In still another approach, anti-backlash gears are used. In application, an anti-backlash gear mates with a standard gear. A typical arrangement consists of two concentric gears, with one gear rigidly held to the shaft and the other gear coupled through a spring with a set pretension. The gear teeth of both anti-backlash gears sandwich the standard gear, and depending on the direction of force, independently act as the load path of the gear train. Even when the force is in the direction of the gear held by the spring, as long as the pretension is not overcome, the motion of the anti-backlash gear perfectly tracks the previous gear in the gear train, and there is no backlash. When the preload is overcome, however, the position of the anti-backlash gear becomes a function of the loading, the spring constant, and the preload, which has been viewed as an undesirable behavior.
Some applications can tolerate a compliant drive, and even some applications, such as series elastic actuators, require it. A typical approach in such applications is to find a gearbox with an appropriate level of backlash and an independent compliant system, and combine them. Although the gearboxes are chosen based on their relationship between input motion and output motion, the whole system behaves with an input motion and an output force.